The proton-translocating sector (FO) of the H ion-pumping ATPase of E. coli is composed of three nonidentical subunits. The specific function of each subunit and the arrangement of these subunits in the membrane is not known. Mutants have been isolated in which the FO-sector is defective. These mutants are being characterized by genetic complementation to determine which gene, specifying an FO protein, is altered. The FO of these mutants is being analyzed to determine whether FO is assembled properly and which subunit is altered by mutation. The effect of the mutational alteration on (1) H ion-translocation by FO, and (2) on the binding of the ATPase component of the complex to FO is being studied. The long-term goal is to identify the molecular components of FO that are essential in H ion-translocating function and in the coupling of H ion-translocation to ATP synthesis from ADP and Pi.